The present invention relates to a furnace for producing a melt for mineral wool production
In mineral wool production, minerals of silicon and metal oxides or carbonates and/or slag are used as raw material. The raw material is melted in a furnace and the melt is fed into a fiberizing unit, which converts the mineral melt into fibres. During the fiberizing, a binding agent is added, which upon a thermal treatment fixes the fibres into each other so as to provide a shape permanent product. In order to provide a product of good quality, it is vital that the melting be regular and that the viscosity of the melt and the flow from the furnace to the fiberizing unit be constant.
The most commonly used type of melting furnace is a water-cooled cupola furnace, which is charged with a mixture of mineral raw material and coke. An inlet for combustion air is provided in the cupola furnace, and the combustion air is usually preheated to about 500.degree. C. Stone raw material of a basalt or diabase type melts at a temperature of about 1200.degree. C. The melt is overheated in the furnace so as to flow out of the furnace at about 1450.degree. C. Due to the coke intermixed with the mineral raw material, the melting in the cupola furnace takes place in a reducing atmosphere. The exhausted flue gases then typically contain 8 to 10% of unburned carbon monoxide (CO) and a small amount of hydrogen sulphide (H.sub.2 S) and also sulphur dioxide (SO.sub.2).
A serious drawback of the cupola furnaces is that they in most cases for environmental reasons have to be equipped with a gas purifier, which separates the dust and afterburns the flue gases, and which also includes preheating of the combustion air. Such a gas purifying and afterburning equipment is usually more complicated and expensive than the cupola furnace for the melting. Another drawback involved with the reducing atmosphere when using cupola furnaces charged with coke is that the iron oxide present in the raw material is reduced into metallic iron, whereby the furnace has to be equipped with a device for iron tapping. The iron tapping, which takes place every four hours, also causes a production stop of about 15 minutes.
In addition, cupola furnaces using natural gas as additional fuel have been manufactured for energy saving reasons, whereby a maximum of 25 to 30% of the coke has been replaced with gas. However the problem of incomplete burning remains, requiring afterburning, waste gas purification and iron tapping, which again involves a production break.
Other types of melting devices are electric furnaces of a tank type, in which the melting energy is supplied by three graphite electrodes fed with 150 to 200V alternating current and 10 000 to 20 000 A current. The amounts of exhaust gas from an electric furnace are normally small enough not to need purifying. Besides this environmental advantage, the electric furnace also has the advantage of allowing more freely the choice of raw material (finely divided) and also yields a more regular melt flow and temperature than the cupola furnace. However, the electric furnace has the drawback of incurring about four times higher equipment expenses than a cupola furnace with gas purification, and that of requiring a very refined and expensive energy form.
Moreover, gas or oil heated melting tanks of a similar type as the ones used in glass industry are being used, possibly with a 5 to 10% energy addition through molybdenic electrodes. Gas heated tanks incur somewhat more expensive equipment costs than electric furnaces of a tank type, and comprise approximately twice as large a melt bath surface for the same capacity. Gas and oil have the advantage of being easily available fuels, whereas gas or oil heated tanks have the drawback of a lining endurance of only 3 to 4 years and the reconstruction costs represent about 30 to 40% of a newly constructed tank.
All the above furnace types have their own advantages and drawbacks, preventing any of them from being ideal, however the trend today is to use electric or gas heated furnaces for environmental reasons (mainly sulphur).
As noted above, the latter furnaces have the drawback of being markedly more expensive than cupola furnaces with flue gas purification, being at the same time less flexibly adaptable to a discontinuous operation during weekends or for instance 1 or 2 shift operation. The cupola furnace comprises only a few hundred kilograms of melt at its bottom, whereas the tank furnaces have a melt quantity of several tens of tons. For that reason the cupola furnace may run down and be emptied of melt very rapidly without noticeably loosing production time. Likewise, the cupola furnace may be run up in a very short time in 1 or 2 shift operation, usually in about 1 hour. On the other hand, the running up of tank furnaces lasts 1 to 3 days, signifying that they cannot be made flexible in view of a discontinuous operation.
GB patent specification 1 326 884 discloses among others a gas heated iron melting furnace of a cupola furnace type, in which the melting takes place in a shaft, the bottom part of which comprises a water-cooled grate, which supports the iron raw material to be melted, and a bed of ceramic graphite blended filling bodies, which slowly melt down and are mixed with the melted slag, the iron melt being simultaneously carburized. The iron melt deriving from such a furnace has a temperature of about 1350.degree. C. and requires the temperature to be raised to the proper casting temperature, which usually is done in a separate induction furnace.